Electrostatic precipitator sample counter



Dec. 8, 1959 R. D. THOMAS AL ELECTROSTATIC PRECIPITATOR SAMPLE COUNTER Filed Oct. 26, 1956 3 Sheets-Sheet l 0/795 BYEDWHED l4. BUB/CE INVENTORS. EOBEET 0. TH

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ELECTROSTATIC PRECIPITATQR SAMPLE COUNTER Filed 001;. 26, 1956 3 Sheets-Sheet 2 IN VEN TOR5. EOBEET D. THO/1H5 EOWHBD 9. UBKE QM "LVWI;

United States Patent f ELEcTRosTATItgREcrPrrAToR SAMPLE UNT ER Robert D. Thomas, Fairborn, Ohio, and Edward A. Burke, South Boston, Mass., assignors to the United the Air Force Application October 26, 1956, Serial No. 618,658

8 Claims. (Cl. 250-83).6)

(Granted under Title 35, US. Code (1952), see. 266) .States of America as represented by the Secretary oftogether with an ionization chamber or' counter for rapidly and accurately making quantitative determinations of radioactive particulate concentrations in air, with negligible decay in the half-life of the radioactive partides, and more particularly to a method and a device which permits rapid, accurate and reproducible quantitative analysis of radioactive particulate materials in. air, such as the air breathed in work areas where radioactive materials are being processed and for comparable determinations. v I As background to make the 7 present invention, as claimed, clearly understandablain work areas where radioactive materials are processed, the health of workers maybe impaired by physiologically excessive concentrations of dustparticles which are laden with radioactive material, of which alpha, beta and gamma radiations may be taken as being illustrative. p

4 In-the making of past determinations quantitatively by an electrostatic precipitation method, a commercially available MSA Electrostatic Sampler, which is. manufacturedand distributed by the Mine Safety Appliance Cornpany of Pittsburgh, Pennsylvania, has been used. Cylindrical aluminum tubes form part of the equipment of this type of electrostatic sampler device and are prepared for the collection of radioactive particulate material on the surface of a thin clean aluminum foil which is carefully and smoothly spread against the inner surface of the'tube over all o f'th e tube internal'area. One'end of the tube is then inserted into a blower housing-part of the electrostatic sampler and a switch on the blower housing grip is actuated, causing air which is to be tested to be drawn through the tube at the rate'of 3 cubic feet per minute for one hour, for a total test sample air volume of 180 cubic feet. Solid particles floating as dust in the air being tested are caused to adhere to the aluminum foil within the sampling tube during the above illustrative one hour sample collecting time by a potential difference between an inner electrode and the aluminum foil within thetube of from 12 to 15 thousand volts direct current depending upon the relative humidity of the air under test.

At the end of the prescribed time for collecting asample, the air sampling tube is removed from the blower housing of the electrostatic sampler and the aluminum foil is very carefully removed from the sampling tube and is cut with shears into squares of-a size to fit into a sampling chamber in another commercially available piece of equipment, such as a Proportional Counter Converter model PC-C 10 marketed by the Nuclear Measurements 'Co'rporation'of Indianapolis, Indiana. In the use of this commercially available proportional counter converter,

2,916,626 Patented Dec. 8, 1959 ice rial adhering to that piece of aluminum foil. This procedure is repeated for each piece of aluminum foil into which the particle hearing test foil has been subdivided. The count readings for each piece of foil then are addedtogether in arriving at the total quantitative radioactivity content of the total air test sample. I

The described total air sample radioactivity content de-- termination consumes about another hour of time, during which time the radioactivity of the radioactive particles on the pieces of aluminum foil suffer their particular char-- acteristic decay rates which introduce errors. A further source of error is the particle mechanical loss resulting; from the handling and the cutting ofthe aluminum foil.

The design and the operation of dynamic condenser electrometers areadequately explained in the Review of Scientific Instruments, published in May 1927, volume 18, pages 298 to 314, inclusive. -A second information source is Radiological Monitoring Methods andlnstrumentsj issued April 7, 1952, by the National Bureau of Standaids, available from the Superintendent of Documents, Washington 25, D.C. f l The' present invention uses the same mine safety electrostatic precipitator which is described above for, collecting samples. This invention .has as its advantageous novelty the complete avoidance of the use of aluminum foil, together with the difiiculty encountered in the required smooth, careful and complete application of the aluminum foil to the insidesurface of the air sampling tube. Also avoided are the removal, the handling and the cutting of the aluminum foil taken from the tube. Also are avoided the running of separate determinations on the resultant pieces of aluminum foil. The invention efie cltively minimizes the mechanical lossof particles of radioactive material from the sample collected and the loss of radioactivity carried by the particles on the foil from aging. The present inventionfurther permits accurate data reproducibility in quantitative determinations made in a matter of minutes, as compared with an om r more in the making of each determination by the previously established method. The procedure introduced by this invention eliminates mechanical losses, reduces "the radioactive half-life losses of the particulate material, and materially conserves time in the actual making of'the determinations. -The improved accuracy of thequantita- 'tive determinations jconternplatedin the practice ofthe present invention is further attested by the dependable reproductions of deter minations madethereby. f The general statement of the matter and the substance of the presentinvention' commensurate, consistent and in harmony with the claims appearing in the patent'issuing hereon, comprises in nature a method and apparatus "for making quantitative analyses of the radioactivity content .of the particulate matter in an air sample. i .1 In substance the present invention comprises a'method for analyzing air for itsradioactivity content and 'comprises an ionization chamber consisting of a hollow cylin= drical air sampling tube, a cap closing one end of the tube, and a socketbase closing'the end of the tube remote from the cap. The socket base supports an anode electrode Wire which extends axially along thecenter of the tube and is supportedat oneend in the socket base.

A negative electrical contact ionizationfchamber. comprises a closed tube entrapping iairin its interior and insulated from an anode collector each piece' 'of aluminum foil is separately inserted and a a reading takenof'the magnitude of the radioa'ctivemateelectrode extending axially inside the tube. I

"The generalstatementobject ofthe present invention commensurate with the invention as claimed, is the .pro'- vision of a substantiallydirect reading instrumentwhich provides quantitative determinations of theradioactivity of particulate materials 'in'air'," such as the 'airrouadtn Work areas where radioactive materials are handled and embodiment of the present invention.

Fig. l is a perspective view of an. ionization chamber part of the present invention. comprising. a tube; a cap and a base in assembled relation.

Fig. 2 is a perspective exploded viewof the separable parts of the device shown in Fig. 1.

Fig. 3 is a section taken along the line 33 of Fig. l; and

Fig. 4 is a fragmented view of the ionization chamber shown in Fig. I, mounted with commercially available instruments for reading. radioactivity in microcuries per milliliter of air sample.

In the accompanying drawings the ionization chamber shown consists of two conductors which are insulated from each other and which have an air gap therebetween. The outer conductor is a hollow cylindrical tube which is dimensioned to be used interchangeably with similar tubes supplied as parts of the Mine Safety. Appliance Electrostatic Precipitator, Model 1175, produced and marketed by the Mine Safety Appliance Company of Pittsburgh, Pennsylvania. The ionization chamber outer conductor tube 1 has extending axially along the center thereof an anode collector electrode 2. The tube 1 is closed at one end by removable cap 3 and is closed at its opposite end by an adaptor base 4 which supports the inner electrode 2'. The tube 1, preferably makes a slip fit with the cap 3 at one end and with a socket of the adaptor base. 4 at its opposite end. The distance from the free end of the inner collector electrode 2 to the cap 3 is the same or less than the distance from the tube 1 to the electrode 2 for the maintaining of a uniform electrostatic field between the electrodes 1 and v2 which are of opposite polarity.

The tube 1 illustratively may be made of aluminum and the inner collector electrode 2 may be a wire made of copper, brass or the like. The ionization chamber inner conductor or collector electrode 2 is attached at one end to a central contact plate 5, also of copper, brass or a similar electrical conductive material. The wire 2 and the plate 5 are shown in the drawings to be attached by .solder 6 to an anchor plate 7. It will be apparent that the anchor plate 7, which is secured by screws 8 to an insulator 9 of polymerized methyl methacrylate or the like, may be assembled into a single unit if desired. The insulator 9 must have an adequate electrical resistance for its intended use, such illustratively as 10 ohms, which is a characteristic of polymerized methyl methacrylate merchandized as Lucite. Plexiglas is an alternate.

The insulator 9, illustratively may be a circular disk as shown. One side of the plastic insulator 9 has permanently attached thereto a circular metal plate 10 with a flange 11 dimensioned to receive one end of the tube 1 preferably inwardly thereof. The end of the tube 1 may tachment thereto of a lead conducting direct current electricity of a polarity opposite to that applied to the central contact 5. The flange 15, which is concentric with the central connector 5, is dimensioned and shaped to engage a turret contact of a. commercially available vibrating reed electrometer. A suitable representative electrometer is available from the Applied Physics Corporation of Pasadena, California.

The vibrating reed. type of electrometer preferably used in the practice of the present invention measurescurrents and comprises a vibrating reedwhich produces avarying capacitance. The varying capacitance is amplified. and is applied to a reversible motor which acts as a synchronous detector. The motor output drives the contactor of a potentiometer where a signal is simultaneously recorded and balanced.v The final determinations are commonly expressed in microcuries per milliliter of air sample. For purposes of conversion onemicrocurie equals 10- curie and one millicurie equals l0 curie.

Preparatory for the collection of radioactive, particles on the inside wall of the tube 1, the separable par-ts of the ionization chamber are cleaned thoroughly, as with dilute nitric acid, distilled. water and an uncontaminated air stream, or. the like. One end of the tube. then is in serted into the blower housing of the electrostatic precipitator, which. incorporates a central electrode. The blower ofthe electrostatic precipitator is then energized and air is drawn through the tube 1 at a predetermined velocity and for a predetermined period of time to provide the passage of a predetermined volume of air through the tube. As particulate material enters the air sampling tube 1 the. high electrostatic field potential between the central electrode and the inner surface of the tube 1 causes the deposit on the inner surface of the tube 1 of airborne particulate materials- The cap 3 is then placed on the unattached end of the tube 1 and the opposite end of the tube 1 is removed from the blower housing and is inserted into flange 11 of the base 4 in such a way that the electrode 2 extends axially centrally inside of the tube 1.

The tube 1 contains radioactive particulate material which is adhered upon the inner surface of the tube. The particulate radioactive material exerts an ionizing effect on the air within the tube. The assembled ionization chamber comprising the tube 1, the cap 3 and the base4 is then mounted on the turret 20 of a vibrating reed device 21, as is shown in Fig. 4 of the accompanying drawlngs. The components in Fig. 4, to which the ionization chamber is to be connected, remain connected together for an hour or more for the soaking-in efiect, prior to the making of a determination. The soaking-in effect is prolonged for a sufiicient period of time for the substantially complete orientation of the insulator 9 in regards to the negative and positive charges applied to the respective anode and cathode of the ionization chamber. When so assembled the downwardly extending flange 15 and the central contact 5 of the base 4 bear a positive charge which is supplied over a lead 23 from a direct current source 22, of illustratively 300 volts. The nega tive terminal of the direct current source 22 is connected through a lead 24 to a contact 16 and impresses a 300 volt direct current of negative polarity through the upper disk 10 to the tube 1.

The turret 20 of the vibrating reed device 21 is equipped with a lever 25 which can be moved along a slot 26. The operation of the lever 25 selectively places in circuit any one of three resistors of magnitudes 10 10 and 10 ohms housed within the turret. The turret lever 25 is in its ground position when preliminary adjustments are made. Set screws 27, 27', etc. thread through the collar of the turret 20 and bear firmly against the outer surface of the flange 15 which depends from the disk 14 part of the counter base 4. In assembling the ionizing chamber base 4 with the turret 20, the anode contact plate 5 at the lower end of the wireelectrode 2 engages to depress an upwardly spring loaded contact at the center of the turret. -The potential of the center electrode 2 is changed in in'ag'nitude bythe collection of ions produced in the surrounding air medium by the nuclear particles emitted by the radioactive particulate material adhering to the inside wall of the tube 1. The radioactive material within the tube produces ionization of the entrapped air at a constant rate. Changes of potential on the collecting electrode2 are proportional to the degree of air ionization. Potential loss from the ionization chamber is restored at a constant rate from the battery 22 during the collection of electrons on the collecting electrode 2. The loss of potential supplied by the battery 22to thecollection electrode 2 of the ionization chamber produces current at the contact plate 5 driven to or 10 amperes. The current so produced passes through the vibrating reed device 21 over the connecting cable 30 to the electrometer 31 where it is measured and then over the cable 38 to the recorder 39 where current changes on' a time basis are recorded.

The electrometer 31 is provided with an on-otf switch 32 from a power supply line lead 33 of 110 volts potential which energizes the electrometer. The electrometer 31 is provided with two polarity switches 34 and 34' which are switched on after the equipment is warmed up. The switch 34 impresses a selective polarity on the meter 36. The switch 34 impresses a selective polarity on a galvanometer within the recorder 39. These switches 34 and 34' are moved just before a determination is made. The meter 36 is zeroed prior to each run by a zero adjusting dial 35. The meter 36 is provided with a hand 37 which travels along a scale 38 which is calibrated in millivolts. A'range selector 40 establishes the multiplication factor for the meter readings on the meter 36. The lead 38 conducts output from the electrometer 31 to the recorder 39. The range selector 40 is energized from the electrometer on-ofi switch 32.

The electrical effects so recorded are accurate indications quantitatively of the relative concentrations of radioactive particles in the total air sample drawn through the tube 1 and under analysis and consequently the relative degree of physical hazard to the workers who are employed within the test area from which the air sample was taken. The calibration of sensitivity for the equipment described herein in the detection of radioactive particulates, illustratively has been determined to be 6.325 alpha counts per minute per millivolt. The counter has a very high counting efficiency for all radioactive particulates.

The counter has potential use in radiation facilities in general, such as with hot cells, with atomic reactors, as an instrument for making safety checks in atomic powered vehicles such as submarines or the like. The counter is particularly well adapted for the making of daily monitoring checks of the air in work areas where radioactive materials may be processed, stored or the like.

It is to be understood that the tube, the collector electrode wire, the cap and the base which comprise the physical parts of the present invention, together with procedural steps by which these parts are used with commercially available equipment, and also the method or the process steps by which determinations are made by the use of the described equipment, are submitted hereby as successfully operating embodiments of the present invention and that comparable modifications may be made in both the parts of the device and in the steps of the processes described herein without departing from the scope of the present invention.

What we claim is:

1. The method for quantitatively determining radioactive particulate material inclusive of alpha particles and deposited by the electrostatic precipitation method on the inside of a tubular cathode with removable cap adapters making slip fits with both ends of the cathode and through which cathode a predetermined volume sample at ambient temperature and pressure of contaminated air is drawn at a constant velocity, with both cap adapters removed from the ends of the cathode making a static state air ionization chamber by enclosing a static air medium within the tubular cathode with the adapters closing the opposite ends of the tubular cathode, and measuring the ionization produced in the air medium within the ionization chamber acted on by the radioactive particulate material collected from the air sample. I

2. The method for making quantitative determinations of radioactive particulate material inclusive of alpha particles in air using a static air filled ionization chamber contoured as a hollow cylinder with end caps making slipfit engagement with both ends of the cylinder and the chamber containing radioactivity bearing particulate material accumulated within the ionization chamber by the electrostatic precipitation method from a known volume of air, forming an ionization chamber by enclosing a static predetermined volume of air medium within the tubular cathode provided with an anode extending axially centrally thereof and with both end caps slipped on the ends of the hollow cylinder, impressing a substantially constant direct current potential between the anode-cathode electrodes of the ionization chamber in maintaining the anode at a substantially fixed potential and at a substantially constant current output, and measuring the current output from the ionization chamber as being indicative of the quantitative determination of the amount of radioactivity contained in the deposited material collected from the test air sample by the electrostatic precipitation method.

3. An ionization chamber comprising a hollow cylindrical tube, a cap making a slip-fit on one end of the hollow cylindrical tube, and a base making a slip-fit on the end of the hollow cylindrical tube remote from the cap and the base supporting a collecting electrode extending axially and substantially centrally of the tube, with the cap and the base slipping axially on the opposite ends of the tube and thereby cooperating in closing in freely removable slip-fit the opposite ends of the tube for entrapping air in its static state therein.

4. The ionization chamber defined in the above claim 3 wherein the tube is a hollow cylinder relcasably closed at one end by the cap when advanced by axial movement along the hollow cylinder and at the other end by the collecting electrode supporting base when advanced by axial movement along the hollow cylinder in a capping action therewith and the base, and the base comprising a dielectric insulator of a magnitude of 10 ohms electrically between the tube and the collecting electrode.

5. The method of running a quantitative determination on the radioactive particulate material content in contarninated air by passing a predetermined quantity of contaminated air between an anode and a clean cathode of an electrostatic precipitator for depositing the particulate material content of a test air sample on the cathode, modifying the precipitator cathode by enclosing the ends thereof by sliding axially thereover clean closure members resulting in an ionization chamber with a predetermined volume of static air entrapped therein and with an indeterminate quantity of particulate material adhering to the inner surface of the cathode, impressing upon and maintaining at a predetermined electrical potential the anode and the cathode within the ionization chamber, and measuring quantitatively the air ionization by the radioactive particulate material Within the ionization chamber.

6. The method of running a quantitative determination on the radioactive particulate material content in contaminated air by passing a predetermined volume of air between an anode and a hollow and cylindrical clean cathode serving as an electrostatic precipitator with a predetermined potential maintained between the anode and the cathode for accomplishing the deposition and the retention on the inner surface of the cylindrical cathode of an unknown quantity of radioactive particulate material from the contaminated air which has passed therebetween during a predetermined period of time, the entrapping of an unknown quantity of radioactive particulate material retained on, inner surface of the cathode with a known volume of air. maintained in the static state between the anode, and the cathode by closing both ends of the cathode with clean caps making axially sliding engagement therewith to serve with the cathode as av closed ionization chamber, applying a predetermined electrical potential to the chamber electrodes for a predetermined period of time for accomplishing the ionization of the entrapped air by the deposited particulate materials within the ionization chamber until a current reading is obtained to indicate the; degree of ionization which is relative to the amount of radioactive materials deposited in the chamber.

7. The method described in the above claim 6 wherein the predetermined period of time for purposes of standardization is one hour and the predetermined electrical potential is 300 volts direct current.

8. The method for quantitatively determining radioactive particulate material deposited by the electrostatic precipitation method on the inside surface of a tubular cathode through which a predetermined volume sample of contaminated air'is drawn at a constant velocity, a static stateair ionization chamber byenclosing a con stant volume of static air Withinthe. tubular cathode with clean cup shaped slip on adapters closing both of the ends of the tube, applying a saturation potentialbetween' References Cited in the file of this patent UNITED STATES PATENTS 2,519,007 Wilson Aug. 15, 1950 2,596,080 Raper et al May 6,1952 2,625,657 Kanne Jan. 13, 1953 2,666,865 Borkowski Jan.- l9,'. 1954 2,755,391 Keyes July 17, 1956 

